1. Field of the Invention
The invention relates to a commutation circuit and a motor driving apparatus, and more particularly, to a sensorless commutation circuit and a sensorless motor driving apparatus for brushless motor.
2. Description of Related Art
Current brushless direct current (DC) motors use sensor to detect the rotating positions of motors. For example, after positions sensors such as hall elements, light encoders, or resolvers have been additionally installed, drivers can then send out suitable commutation signals causing the motors to rotate normally. The advantages and disadvantages of commutation control will directly affect the performance of the motion control such as the position or speed of the rotor. If the resolutions of these sensors are raised in order to increase the precision of commutation control, the production cost will also increase. These sensors also increase the required volume of an apparatus, which further increases production cost.
As products are becoming thinner and lighter, motors are also becoming smaller, with higher precision, and less noisy. Since motors are becoming smaller, the amount of space taken up by position sensors has been a main hindrance when desiring to make products thinner. In addition, factors such as the life-span of a position sensor, temperature, and interference sensitivity, decrease the reliability of sensor components, which in turn affects the reliability of commutation control. As such, the conditions and environment of where a motor is applied is further limited.
Thus, sensorless driving apparatuses can decrease the time and effort motor vendors spend on for finding the precise position of rotor. Based on the aforementioned restrictions of sensor components, sensorless driving apparatuses that do not require position sensors have become more attractive, and have great potential in the development of driving technology.
In general sensorless driving apparatuses, a commutation circuit is used to replace the hall elements, to detect the location of the rotor and transmit the commutation signals to the controller, so as to drive and control the brushless motor. The brushless motor requires a pulse width modulation (PWM) signal when driving to modulate and control the rotational speed of the motor. However, when the brushless motor is at the initial startup of an open loop, since the brushless motor is operating under a low speed condition, the commutation signal outputted to the commutation circuit by the PWM signal is viewed as a significant interference signal. This causes the controller to be unable to determine if the signals received are correct switch signals. Therefore, the brushless motor requires a longer time for ticking over. Only when the rotation speed of the brushless motor is increased, and the effect of the PWM signal towards the commutation signal is decreased, does the brushless motor enter a closed loop control condition, and operate under a stable condition.